1. Field of the Invention
This invention relates in general to timing recovery loops, and in particular, to a digitally controlled timing recovery loop for long range repeaters transmitting data at 1.544 Mbps or 2.048 Mbps.
2. Description of Related Art
Long range repeaters are typically used in the transmission of data at 1.544 Mbps or 2.048 Mbps. Often, a large number of repeaters (as large as 200) are connected in tandem, so it is extremely important to minimize accumulated intrinsic jitter from each repeater. In order to minimize the accumulated jitter at the end of a chain of repeaters, the intrinsic jitter of each individual repeater must be very low.
Most long range repeaters have adaptive equalizers in their front ends to reconstruct the received signal from the transmission channel. Since the transmission medium has limited bandwidth, the signal appears distorted at the end of the channel. Distortion on a pulse causes the pulse to reduce in amplitude and spread out in time. Since one pulse smears into another, it becomes difficult to extract timing information.
The equalizer adjusts the gain and frequency response of its filter to negate the effect of the channel and bring the pulse stream back to a distortion-free state. Since the equalizer cannot remove 100% of the distortions in a pulse, residual distortion, called Inter-Symbol Interference (ISI), is left on the equalized pulse. This ISI causes the pulse to shift, and distorts both edges thereof somewhat symmetrically about the peak of the pulse.
The distortion of the pulse is a function of the type of data bits transmitted before a current bit. Typically, marks are transmitted as pulses and spaces are transmitted as no pulses. Thus, if a previous bit was a mark and its ISI affects the current bit, the current bit will move by the ISI induced by the previous bit. However, if the previous bit was a space, then the current bit is not affected. This results in data dependent, ISI-induced, intrinsic jitter.
ISI affects both edges of a pulse, by moving it in time and reducing its amplitude. If only one edge of the pulse is used to gain timing information from the data stream, then a timing recovery loop in the repeater would pass ISI-induced jitter through. This would make the intrinsic jitter too high. The major part of ISI distortion occurs in pulse shape distortion rather than in pulse position distortion.
In addition to minimizing intrinsic jitter, long range repeaters are also required to tolerate a relatively large amount of incoming jitter. This requires that the timing recovery loop in the repeater adapt its clock quickly to follow the jittered data without losing the data. This produces a design constraint inconsistent with the requirement of low intrinsic jitter.
Whatever the merits of prior timing recovery loops, they do not achieve the benefits of the present invention.